1. Field of the Invention
The invention relates to a coil spring of a valve, in particular, a coil spring of a valve that is provided in a passage through which fluid is delivered from a tank by a pump, and that adjusts the pressure of the fluid, for example, a valve that adjusts the pressure of fuel supplied from a fuel tank to the injector of an internal combustion engine by a fuel pump.
2. Description of the Related Art
A fuel supply system of a vehicle or the like, which is a system for supplying fluid, includes a fuel passage 5 that connects a fuel tank 1, a fuel pump 2, and the injectors 6 of an engine, as shown in FIG. 17. The fuel in the fuel tank 1 is supplied to the injectors 6 of the engine via filters 3 and 4 by the fuel pump 2 provided in the fuel tank 1. In this case, the pressure of the fuel in the fuel passage 5 fluctuates according to, for example, the operating state of the engine. Therefore, a pressure adjusting valve 8, which adjusts the pressure of the fuel to a predetermined value, is provided in the fuel supply system. The pressure adjusting valve 8 is provided in a branch passage 7 that extends from the fuel passage 5. When the pressure in the fuel supply system is equal to or above the predetermined value, the pressure adjusting valve 8 is opened to return the fuel to the fuel tank 1.
FIG. 16 shows an example of a conventional pressure adjusting valve 10. The pressure adjusting valve 10 includes a case 11, a valve element 14, and a coil spring 16. The case 11 is a member that forms the framework of the pressure adjusting valve 10. The case 11 includes a fuel inlet 12, and a fuel outlet 13. A valve seat 18 is formed near the fuel inlet 12 on the inner surface of the case 11. A spring bearing portion 17 is formed in the bottom portion of the case 11.
The valve element 14 includes a cylindrical portion 15. The valve element 14 is moved upward and downward in the case 11 while the cylindrical portion 15 is moved along the inner surface of the case 11. The coil spring 16 is provided between the inner bottom surface of the valve element 14 and the spring bearing portion 17. The coil spring 16 constantly presses the valve element 14 toward the fuel inlet 12 so that the end of the valve element 14 contacts the valve seat 18, and communication between the fuel supply system and the fuel tank 1 is interrupted. However, when the pressure of the fuel in the fuel supply system is equal to or above the predetermined value, the coil spring 16 is pressed downward by the valve element 14. As a result, the valve element 14 is moved away from the valve seat 18, and accordingly, excess fuel is returned into the fuel tank 1. In the pressure adjusting valve 10, the cylindrical portion 15, which is moved along the inner surface of the case 11, is provided. Therefore, the axis of the valve element 14 is not deviated from the axis of the valve seat 18, and appropriate sealing is provided between the valve element 14 and the valve seat 18. On the other band, the axis of the valve element 14 is slightly inclined due to fit dimension tolerance and shape tolerance. Accordingly, the valve element 14 is not moved smoothly. As a result, the pressure of the fuel at a valve opening time differs from the pressure of the fuel at a valve closing time. Also, the fit portion, which is moved along the inner surface of the case, needs to be formed with high dimension accuracy. This increases the cost (refer to Japanese Patent Application Publication No. 8-320074 (JP-A-8-320074).
To solve such a problem, for example, a poppet valve, which includes a conic valve element as shown in FIG. 1, is available. In the poppet valve, the valve element 34 does not include a member that is moved along the inner surface of a case, and the valve element 34 is supported by a coil spring 40. When the valve element 34 opens a fuel inlet, the valve element 34 is supported by one end of the coil spring 40 such that the valve element 34 is floated. Also, when the valve element 34 closes the fuel inlet, the coil spring 40 makes the valve element 34 contact a valve seat 31b. 
In such a poppet valve, the valve element has a conical shape. Even if the axis of the valve element is slightly deviated or inclined with respect to the axis of the seat valve when the valve element contacts the valve seat to close the fuel inlet, appropriate sealing is provided. Thus, although measures are taken, for example, so that the quality of the material of the coil spring is uniform, and the entire coil spring has a uniform diameter, no particular consideration has been given to a measure to be taken so that a load is uniformly applied to the entire circumference of the coil spring when the coil spring extends or contracts. For example, no particular consideration has been given to how the upper and lower effective winding ends in the upper side and lower side of the coil spring should be arranged in the plan view. Accordingly, when the conventional coil spring extends or contracts, the axis of the coil spring tends to be deviated or inclined with respect to the axis of the valve seat.
In recent years, it has become necessary to more accurately control the supply of fuel in a vehicle or the like. Accordingly, it has become necessary to more accurately adjust the pressure of the fuel using the pressure adjusting valve. However, as described above, the axis of the conventional coil spring tends to be deviated or inclined with respect to the axis of the valve seat when the conventional coil spring extends or contracts. If the axis of the coil spring is deviated or inclined with respect to the axis of the valve seat when the coil spring extends or contracts, the axis of the valve element is also deviated or inclined with respect to the axis of the valve seat when the valve element opens or closes the fuel inlet. If the axis of the valve element is deviated or inclined, for example, the pressure of the fuel when the valve element opens the fuel inlet differs from the pressure of the fuel when the valve element closes the fuel inlet, or appropriate sealing is not provided between the valve element and the valve seat. As a result, it is not possible to sufficiently satisfy the need of accurately adjusting the pressure of the fuel.